Ink composition and method for use thereof in the manufacturing of electrochemical sensors

ABSTRACT

An ink composition for manufacturing electrochemical sensors in accordance with the present invention includes graphite, carbon black, a resin and at least one solvent (e.g., at least one solvent with a boiling point between 120° C. and 250° C.). The ink composition has a weight ratio of graphite to carbon black is in a range of from 4:1 to 1:4 and a weight ratio of a sum of graphite and carbon black to resin in a range of from 10:1 to 1:1. Also, a method for manufacturing an electrochemical sensor includes transporting a substrate web past at least one print station and printing at least one electrochemical sensor electrode on the substrate web at the print station(s). The printing is accomplished by applying an ink composition to substrate web, wherein the ink composition includes, graphite, carbon black, a resin and at least one solvent. In addition, weight ratio of graphite to carbon black in the ink composition is in a range of from 4:1 to 1:4 and a weight ratio of a sum of graphite and carbon black to resin is in a range of from 10:1 to 1:1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to ink compositions and theirassociated methods and, in particular, to ink compositions for use inmanufacturing electrochemical sensors and their associated methods.

2. Description of the Related Art

SUMMARY OF THE INVENTION

An exemplary embodiment of an ink composition for manufacturingelectrochemical sensors in accordance with the present inventionincludes graphite, carbon black, a resin and at least one solvent (e.g.,at least one solvent with a boiling point between 120° C. and 250° C.).The ink composition has a weight ratio of graphite to carbon black in arange of from 4:1 to 1:4 and a weight ratio of a sum of graphite andcarbon black to resin in a range of from 10:1 to 1:1.

An exemplary embodiment of a method for manufacturing an electrochemicalsensor according to the present invention includes transporting asubstrate web past at least one print station and printing at least oneelectrochemical sensor electrode on the substrate web at the printstation(s). The printing is accomplished by applying an ink compositionto substrate web. The ink composition which is applied includesgraphite, carbon black, a resin and at least one solvent. In addition, aweight ratio of graphite to carbon black in the ink composition is in arange of from 4:1 to 1:4 and a weight ratio of a sum of graphite andcarbon black to resin is in a range of from 10:1 to 1:1.

BRIEF DESCRIPTION OF DRAWINGS

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription that sets forth illustrative embodiments, in which theprinciples of the invention are utilized, and the accompanying drawingsof which:

FIG. 1 is a flow chart illustrating a sequence of steps in a processaccording to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Once apprised of the present disclosure and the disclosure ofprovisional patent application No. 60/436,683, which is hereby fullyincorporated by reference, one skilled in art will recognize that avariety of ink compositions (also referred to as inks or carbon inks)can be utilized in processes for manufacturing electrochemical sensors(e.g., web-based processes according to the aforementioned provisionalpatent application). However, ink compositions according to embodimentsof the present invention are based on the recognition that it isparticularly desirable to employ ink compositions that (i) provide for aprinted electrode of a manufactured electrochemical sensor to possessbeneficial electrochemical and physical characteristics (such as, forexample, electrochemical characteristics that are essentially equivalentto those provided by a batch manufacturing process and/or a desirableoverpotential, electrochemical surface area, resistance, capacitance,and stability) and (ii) is compatible with relatively high-speedcontinuous web processing techniques.

For an ink composition to be compatible with high-speed continuous webprocessing techniques, the ink composition should be dryable in a dryingduration (time) that does not limit the speed of the continuous webprocess (e.g., a short drying duration in the range of 30 seconds to 60seconds). Such a short drying duration requires more severe (harsher)drying conditions (e.g., the use of 140° C. air at a velocity of 60m³/minute) than a conventional batch process. Unfortunately, when suchsevere drying conditions are used, there is a tendency for the surfaceof conventional ink compositions to bum and/or for a portion of aconventional ink composition that is in contact with a substrate toremain undried. Furthermore, the combination of severe drying conditionsand conventional ink compositions can result in the formation of anelectrode (e.g., a carbon electrode) with undesirable electrochemicalcharacteristics. Therefore, conventional ink compositions typicallyrequire the use of relatively slow drying conditions and a relativelylong drying duration (e.g., approximately 15 or more minutes).

It has been unexpectedly determined that ink compositions according tothe present invention, which include graphite, carbon black, a resin andone or more organic solvents, are particularly useful in themanufacturing of electrochemical sensors. Ink compositions according tothe present invention provide for a printed electrode of a manufacturedelectrochemical sensor to possess beneficial electrochemical andphysical characteristics. The ink compositions are also compatible withrelatively high-speed continuous web processing techniques. Thiscompatibility is due to the relatively high conductivity of the inkcompositions, which enables a thinner printed film (i.e., printedelectrode). In addition, it is postulated without being bound that theprinted electrode is easily dried due to its thin nature and the use ofan ink composition that includes at least one solvent of an appropriateboiling point.

The graphite, carbon black and resin percentages of ink compositionsaccording to the present invention are predetermined such that a weightratio of graphite to carbon black is in the range from 4:1 to 1:4 and aweight ratio of the sum of graphite and carbon black to resin is in therange of from 10:1 to 1:1. Factors which can influence optimizationwithin of the aforementioned ratios are the resulting electrochemicalsurface area, overpotential for oxidizing a redox mediator, as well asthe stability, resistance, and capacitance of a printed carbon film(e.g., carbon electrode).

It is envisioned that ink compositions according to the presentinvention can be used to manufacture carbon films that serve aselectrochemical sensor electrodes. Such carbon films can be used in anelectrochemical glucose biosensor, wherein a current is measured at aconstant potential and the magnitude of the measured current isindicative of a glucose concentration. The resulting current can belinearly calibrated to output an accurate glucose concentration. Amethod of calibrating electrochemical glucose biosensors is to definemultiple calibration codes within a calibration space, in which aparticular calibration code is associated with a discrete slope andintercept pair. For a particular lot of electrochemical sensors, ameasured current output may be mathematically transformed into anaccurate glucose concentration by subtracting an intercept value fromthe measured current output and then dividing by the slope value.

It should be noted that the measured current output, slope and interceptvalues can be influenced by the electrochemical surface area,overpotential for oxidizing a redox mediator, as well as the stability,resistance, and capacitance of the carbon film that serves as theelectrochemical sensor electrode. Therefore, the weight ratio ofgraphite to carbon black and weight ratio of the sum of graphite andcarbon black to resin can be optimized to provide a desired range ofslopes and intercepts.

Any suitable graphite and carbon black known to one skilled in the artcan be employed in ink compositions according to the present invention.In this regard, a carbon black with a surface area of, for example, 20to 1000 m²/g is generally suitable in terms of providing a requisiteconductivity. In general, the conductivity of the carbon black increaseswith the its surface area and a relatively high conductivity carbonblack can be beneficial in terms of providing desirable electrochemicalcharacteristics. Other characteristics of carbon black that aredesirable for use in the present invention are high conductivity, lowsulfur content, low ionic contamination and easy dispersability.Suitable carbon blacks include, but are not limited to, Vulcan XC-72carbon black (available from Cabot) and Conductex 975B carbon black(available from Sevalco). Other types of carbon of carbon black that maybe suitable for the present invention are Black Pearls (available fromCabot), Elftex (available from Cabot), Mogul (available from Cabot),Monarch (available from Cabot), Emperor (available from Cabot), Regal(available from Cabot), United (available from Cabot), and Sterling(available from Cabot), Ketjen Black International Company (availablefrom Ketjen Black), Mitsubishi Conductive Carbon Black (available fromMitsubishi Chemical), Shawinigan Black (available from Chevron PhillipsChemical Company LP) and Conductex® (available from Columbian ChemicalsCompany). Suitable graphites include, but are not limited to, TimrexKS15 carbon (available from G&S Inorganics). The particle size ofgraphite can be, for example, between 5 and 500 μm, but more preferablycan be 15 μm. Other types of graphite that may be suitable for thepresent invention are Timrex KS6 to Timrex KS500 where the numberfollowing the term KS represent the particle size in units of microns.Other characteristics of graphite that are desirable for use in thepresent invention are high conductivity, low ash content, low sulfurcontent and low inorganic impurities.

In general, the surface area of graphite is much less than the surfacearea of carbon black by virtue of graphite's non-porous nature. Forexample, the surface area of Timrex KS15 is approximately 12 m²/g. It istheorized without being bound that the use of graphite in inkcompositions according to the present invention enhances the electrontransfer properties of electrodes manufactured using the inkcompositions. However, an optimized weight percentage of carbon black isneeded in the ink composition in order to increase the overallconductivity of the ink composition. Otherwise, the use of graphitealone would result in a film having a very high electrode resistance.

The electrochemical surface area of a carbon electrode may represent theportion of the carbon electrode that can contribute to the oxidation ofmediator. Graphite, resin and carbon black can have varying degrees ofconductivity and, thus, influence the proportion of the geometricelectrode area that can participate in the oxidation of a mediator. Thegeometric electrode area represents the area of a carbon electrode thatis exposed to a liquid sample. Since the electrode material (i.e., anink composition used to manufacture an electrode) can have an insulatingresin therein, the electrochemical area may be smaller than thegeometric area. In general, the current output of a glucose biosensor isdirectly proportional to the electrochemical surface area. Therefore,variations in the electrochemical surface area may influence the slopeand intercept of the glucose biosensor.

The stability of a carbon electrode is important in designing robustglucose biosensors which are useful to diabetic users. In general,stability of a carbon electrode can be optimized by choosing anappropriate resin and ensuring that sufficient solvent is removed fromthe carbon electrode during drying. It is possible that insufficientlydried carbon electrode can outgas solvent during its storage and thuscause a change in the performance of the resulting glucose biosensor.Furthermore, the stability of the carbon electrode may influence theslope and intercept of the glucose biosensor.

The resistance and capacitance are intrinsic properties of a carbonelectrode and are strongly dependent of the proportions of carbon black,graphite, and resin within the carbon electrode. For example, theresistance of a carbon electrode will increase when a higher proportionof resin or graphite is used in the electrode's formulation. Theresistance of an electrode may influence the electrochemical current ofa glucose biosensor because of the uncompensated IR drop between areference electrode and a working electrode. The capacitance of anelectrode will depend on the ability of an ionic double layer to form atan electrode/liquid interface. The formation of such an ionic doublelayer will influence the magnitude of the measured current. Certainproportions of carbon black, graphite, and resin are likely to enhancethe ability of the ionic double layer to form. Therefore, the resistanceand capacitance of a carbon electrode can influence the slope andintercept of a glucose biosensor.

With respect to an electrochemical sensor of a glucose measuring systemthat includes a working electrode, it is desirable that a relatively lowpotential be applied to the sensor's working electrode in order tominimize the effect of oxidizable interferences that are oftenendogenous to physiological samples. To achieve such a relatively lowpotential, it is beneficial that the material from which the workingelectrode is formed enables the oxidation of ferrocyanide (or otherredox mediator) at the lowest possible potential. This can be achieved,for example, by minimizing the activation energy required for electrontransfer between the working electrode and ferrocyanide (or other redoxmediator). In this regard, it has been determined that the ratio ofgraphite to carbon black is critical in defining (e.g., minimizing) theoverpotential required for the oxidation of a reduced redox mediatorsuch as, for example, ferrocyanide by an electrode of theelectrochemical sensor.

For the above reason, ink compositions according to the presentinvention have a ratio of graphite to carbon black that is in the rangeof from 4:1 to 1:4. Furthermore, a particularly beneficial ratio ofgraphite to carbon black in terms of defining the overpotential has beendetermined to be 2.62:1. It has also been determined that the ratio ofthe sum of graphite and carbon black to resin also influences theoverpotential for oxidizing reduced redox mediator such as, for example,ferrocyanide. And it is for this reason that the ratio of the sum ofgraphite and carbon black to resin is in a range of from 10:1 to 1:1,with a particularly beneficial ratio being 2.9:1.

The resin employed in ink compositions according to the presentinvention can be any suitable resin known to one skilled in the artincluding, but not limited to, terpolymers that comprise vinyl chloride,vinyl acetate and vinyl alcohol. One such terpolymer is VAGH resinavailable from Union Carbide. Resin is employed in the ink compositionas a binding agent and to help adhere carbon black and graphite to asubstrate (such as web substrate) during the manufacturing of anelectrochemical sensor. Additionally, resins such as VAGH will provideflexibility to the printed film, which is especially useful in acontinuous web based processes where printed films must be stable whenrewound into a roll format.

The at least one solvent that is included in ink compositions accordingto the present invention is a solvent in which the resin is soluble andwhich has, for example, a boiling point in the range of 120° C. to 250°C. It is desirable that the boiling point not be less than 120° C. inorder to insure that rapid bubbling does not occur in a printed inkcomposition film when the film is exposed to a drying temperature of140° C. Such rapid bubbling during the drying process could cause theprinted films (i.e., printed electrodes) to have a rough surface whichmay be undesirable. If a solvent's boiling point is greater than 250°C., there is a risk that the ink composition will not sufficiently drywhen exposed to, for example, a drying temperature of 140° C. and an airflow of 60 m³/min for a duration in the range of approximately 30seconds to 60 seconds.

Suitable solvents include, for example, a combination of methoxy propoxypropanol (bis-(2-methoxypropyl) ether), isophorone(3,5,5-trimethyl-2-cyclohenex-1-one) and diacetone alcohol(4-hydroxy-4-methyl-2-pentanone). It should be noted that a combinationof at least two solvents can be particularly beneficial because of apossible decrease in boiling point of the aggregate solvent mixture,i.e., azeotrope mixture. The use of isophorone alone can provide acarbon ink composition with favorable electrical properties. However,the combination of isophorone with methoxy propoxy propanol anddiacetone alcohol can accelerate the drying of the carbon ink. Onceapprised of the present disclosure, one of skill in the art can chooseother suitable solvents with drying properties that are appropriate tovarious drying conditions.

Ink compositions according to the present invention have severalbeneficial properties including being fast-drying while providing forthe manufacturing of an electrode with desirable physical andelectrochemical properties. The ink compositions can be dried quicklyusing relatively severe conditions and are, therefore, compatible withhigh-speed continuous web-based processing techniques. In addition, theink compositions also enable the manufacturing of highly conductivecarbon electrodes even when a relatively thin coating (e.g., a coatingwith a thickness in the range of 5 microns to 20 microns, for example 10microns) of the ink composition is employed. Furthermore, the inkcompositions are of low toxicity, bind well to substrate layers (and toinsulating layers), possess a good print quality and long screen life(i.e., the ink composition does not solidify when used for a long periodin screen printing), and are of low cost.

Ink compositions according to the present invention can be preparedusing any suitable ink preparation technique, including techniques thatare well known to those of skill in the art. In one embodiment of theinvention, the weight % of solids is in the range of 36 to 44% and theweight % of solvent is in the range of 56 to 64%. One factor which helpscontrol the quality and thickness of an ink composition is viscosity. Itshould be noted that the weight % of solids influences the viscosity ofthe ink. In one embodiment of the current invention, the ink compositionhas a viscosity between 11 to 25 Pascal seconds at 50 RPM, and between21 to 43 Pascal seconds at 10 RPM (25° C.). Experimentally, it was foundthat inks with a weight % of solids in the range of 36% to 44% resultedin glucose biosensors having a relatively constant calibration slopewhen preparing glucose biosensors using such inks (see graph below). Itis possible that the more robust calibration slopes was a result of amore uniform electrode thickness resulting from the optimized viscosity.

Carbon ink can be made, for example, by first dissolving 9.65 g of VAGHin an organic solvent made up of 46.53 g of methoxy propoxy propanol,7.90 g of isophorone and 7.89 g of diacetone alcohol in a closed vessel.Next, 7.74 g of carbon black is added to the mixture and then mixed inthe closed vessel. 20.29 g of graphite is then added to the mixture,followed by mixing in the closed vessel. In order to ensure sufficienthomogenization, a triple roll milling is performed on the mixturefollowed by more mixing.

Another embodiment of an ink composition ink composition for use inmanufacturing electrochemical sensors according to the present inventionincludes (i) between approximately 17 and 21% by weight of graphite;(ii) between approximately 6.5 and 8.0% by weight of carbon black; (iii)between approximately 12.4 to 15.2% by weight of a terpolymer resin thatincludes vinyl chloride, vinyl acetate and vinyl alcohol; and (iv)between approximately 55.8 to 64.1% by weight of a solvent mixture thatincludes isophorone, diacetone alcohol and methoxy propoxy propanol.

The ink composition, as well as the ink compositions described above,can be employed in the manufacturing of electrochemical sensors by avariety of processes including, but not limited to, those described inProvisional Patent Application No. 60/436,683. In this regard andreferring to FIG. 1, a process 100 for manufacturing an electrochemicalsensor includes transporting a substrate web past at least one printstation (as set forth in step 110) and printing at least oneelectrochemical sensor electrode on the substrate web at the printstation(s). The printing is accomplished by applying an ink compositionaccording to the present invention as described above to the substrate,as set forth in step 120. As illustrated at step 130, process 100 alsoincludes a step of drying the ink composition that has been applied tothe substrate at temperature of approximately 140° C. with an airflow of60 m³/min. In one embodiment of the invention, substrate web speed maybe 10 m/min

Once apprised of the present disclosure, one skilled in the art willrecognize that processes according to the present invention, includingprocess 100, can be accomplished using methods described in ProvisionalPatent Application No. 60/436,683, which is hereby incorporated in fullby reference.

It should be understood that various alternatives to the embodiments ofthe invention described herein may be employed in practicing theinvention. It is intended that the following claims define the scope ofthe invention and that structures and methods within the scope of theseclaims and their equivalents be covered thereby.

1. An ink composition for use in manufacturing electrochemical sensors,the ink composition comprising: graphite; carbon black; a resin; and atleast one solvent; wherein a weight ratio of graphite to carbon black isin a range of from 4:1 to 1:4; and wherein a weight ratio of a sum ofgraphite and carbon black to resin is in a range of from 10:1 to 1:1. 2.The ink composition of claim 1, wherein the solvent has a boiling pointbetween 120° C. and 250° C.
 3. The ink composition of claim 1, whereinthe solvent includes of isophorone, diacetone alcohol and methoxypropoxy propanol.
 4. The ink composition of claim 1, wherein the resinis a terpolymer that includes vinyl chloride, vinyl acetate and vinylalcohol.
 5. The ink composition of claim 1, wherein the ratio ofgraphite to carbon black is approximately 2.62:1 and the ratio of thesum of graphite and carbon black to resin is approximately 2.9:1.
 6. Theink composition of claim 1, wherein a particle size of the graphite isapproximately 15 microns.
 7. A method for manufacturing anelectrochemical sensor, the method comprising: transporting a substrateweb past at least one print station; and printing at least oneelectrochemical sensor electrode on the substrate at the print stationby applying an ink composition to substrate, wherein the ink compositioncomprises: graphite; carbon black; a resin; and at least one solvent;wherein a weight ratio of graphite to carbon black is in a range of from4:1 to 1:4; and wherein a weight ratio of a sum of graphite and carbonblack to resin is in a range of from 10:1 to 1:1.
 8. The method of claim7 further comprising: drying the ink composition that has been appliedto the substrate at temperature of approximately 140° C.
 9. The methodof claim 7 further comprising: drying the ink composition that has beenapplied to the substrate with an air flow of 60 m³/min.
 10. The methodof claim 7, wherein the drying step has a duration in a range of 30seconds to 60 seconds.
 11. The method of claim 7, wherein the solventhas a boiling point between 120° C. and 250° C.
 12. The method of claim7, wherein the solvent includes of isophorone, diacetone alcohol andmethoxy propoxy propanol.
 13. The method of claim 7, wherein the resinis a terpolymer that includes vinyl chloride, vinyl acetate and vinylalcohol.
 14. The method of claim 7, wherein the ratio graphite to carbonblack is approximately 2.62:1 and the ratio is approximately 2.9:1. 15.The method of claim 7, wherein a particle size of the graphite isapproximately 15 microns.
 16. The method of claim 7, wherein thetransporting and printing steps are accomplished using a continuousweb-based process.